1. Technical field
This invention relates to a noise reduction assembly for an aircraft engine that can be installed on any turbojet equipped with a fan placed inside a nacelle surrounding the engine.
The noise reduction assembly comprises an air intake structure and an annular part that connects the air intake structure onto a fan casing; it may be fixed to the fan casing.
2. Background Art
FIG. 1 very diagrammatically shows a double stream turbojet with a conventional design including a central engine 1 centred on a longitudinal axis, in which the end that is intended to face forwards is located at the left of the figure. By convention, the terms “forward” and “aft” are used throughout this text with reference to the forward and aft side of the engine.
Starting from its forward end, as known, the central engine 1 comprises a compressor, a high pressure turbine driving the compressor, a combustion chamber and a low pressure turbine that in turn drives a fan located forward from the central engine 1. The blades 2 of the fan are located in an annular duct 3 called the “fan duct”, delimited between the outer skin of the central engine 1 and the inner skin of a nacelle 4, arranged coaxially around the central engine 1. By convention, if not mentioned otherwise, the terms “inner” and “outer” are used to denote the position or the orientation of parts with respect to the fan duct 3.
The forward part of the nacelle 4 forms an air intake structure 5 in which the leading edge is streamlined and the aft end is fixed to the forward end of a fan casing 6 arranged around the fan. The fan casing 6 is mechanically and rigidly connected to the structure of the central engine 1 through at least one assembly of arms 7 arranged radially so as to have the best possible control over the existing clearance between the casing 6 and the ends of the blades of the fan 2.
The junction zone between the aft end of the inner wall 8 of the air intake structure 5 and the forward end of the fan casing 6 is shown in more detail in FIG. 2a. 
Noise reduction is one of the priority objectives in the design of turbojets and consequently, it is normal practice to make part of the inner wall of the nacelle 4 in the form of a cellular noise reduction structure: this applies at least partly to the inner wall of the air intake structure 5 and the fan casing 6. Thus, in conventional turbojets, the inner wall 8 of the air intake structure 5 has a cellular sandwich type noise reduction structure away from the zone in which the junction is made, composed of an inner skin 9 permeable to air, an outer skin 10 impermeable to air, and a cellular core 11 inserted between the skins.
For example, the inner skin 9 is made in the form of a perforated plate or a fabric with holes in it made of a material such as carbon or a metal. The outer skin 10 is usually a multilayer composite structure that acts as an acoustic reflector and transmits most of the forces. Finally, the cellular core 11 is usually of the honeycomb type formed from large cells.
The fan casing 6 is usually a metallic part, preferably hollowed out, at least over part of its length, so that a noise reduction structure 12 can be fitted on its inner surface. The structure 12 is then formed mainly from a cellular structure with an inner skin 13 permeable to air on its side facing the fan channel 3.
In order to enable assembly of the air intake structure 5 and the fan casing 6, the fan casing is fitted with an annular outer flange 14 at its forward end. The assembly is made by an annular connection part 15 with an L-shaped section installed around the aft end of the inner wall 8 and fixed to the flange 14 by bolts 16 distributed around the circumference.
In turbojets according to prior art, the connection between the annular connection part 15 and the inner wall 8 of the air intake structure is made by attachment devices such as countersunk screws or rivets (diagrammatically represented by chain dotted lines in FIG. 2), that pass through the aft end of the inner wall 8.
To take account of the fact that most forces transmitted between the central engine 1 and the air intake structure 5 pass through this connection, the structure of the aft part of the inner wall 8 is modified in this zone to be reinforced. Thus, the aft part of the inner wall 8 on which the annular connection part 15 is fixed, has a reinforced inner skin 9′ impermeable to air, an outer skin 10′ also reinforced, and a reinforced cellular core 11′, usually metallic (aluminium) and very dense formed from small cells, often filled with resin so as to resist attachment device crushing forces.
The first consequence of this technique is a complete loss of the noise reduction effect in the splicing zone. It also results in an increase of the mass in the junction zone, risks of corrosion of the metallic cellular core 11′, difficulties in manufacturing (bending of small cells, machining of the shape of the cellular core), difficulties in placing attachment devices through the cellular core 11′ and risks of the aft part of the wall 8 collapsing under load when the attachment devices are put into place.
Finally, the outer dimensions of the inner wall 8 are not very precise due to the fact that it is usually made by successive lay-ups on a mandrel that has the same shape as the inside of the air intake structure 5. Therefore before the annular connection part 15 is fixed onto the aft end of the wall 8, this outer surface has to be remachined and an annular shim 17 has to be inserted.
An improvement to the existing element has been described in document FR-A-2 767 560, and is illustrated in FIG. 2b. In this improvement the aft part of the inner wall 8 used to make the junction with the fan casing 6 does not have a cellular core, the inner skin 9 being applied onto the outer skin 10. Consequently, an annular space is released inside the aft part of the inner wall 8, that is used to house an extension in the forward direction of the cellular noise reduction structure 12, 13 fitted on the inside of the fan casing 6.
However, this element does not correct all disadvantages mentioned above. The characteristics of the sandwich acoustic treatment structure (inner skin, cellular core, outer skin) are specific to the specific features of the acoustic wave to be attenuated. The reduction in the height of cells in the cellular core 12 at the extension in the forward direction causes a difference with respect to the wave to be attenuated. Moreover, an acoustic discontinuity is created by the presence of the junction surface between the two noise reduction structures 9-11 and 12-13, comparable to a weak splice and that causes a loss of acoustic performances.